Field of the Invention
The invention relates to a method for comminuting heat-sensitive feedstock, particularly for the fine grinding of thermoplastics, rubber, caoutchouc, and elastomers to a particle size less than 500 μm, preferably less than 425 μm.
Description of the Background Art
During the comminution of feedstock, a substantial part of the energy to be applied for the comminution is converted to heat. This is caused by cutting, frictional, and impact forces to which the feedstock is subjected during the comminution. In the comminution of heat-sensitive feedstock, to prevent thermal damage it is therefore necessary to cool the feedstock and/or to reduce the heat input by decreasing the production output.
In comminuting devices through which air flows, the feedstock is cooled by the airflow necessary for transporting of the feedstock within the device. This so-called self-generated air can be produced by the device itself and optionally can be supported by a suction fan. If the material is not heat-sensitive, the self-generated air flow inherent in conventional devices is sufficient to cool down the comminuting tools so greatly that any adverse effects on the feedstock are eliminated.
Problems occur on a regular basis when heat-sensitive feedstock is to be comminuted. Especially when materials with a low softening point such as, for example, thermoplastics, are to be comminuted, the operators of conventional devices face a difficult task. In order to achieve the highest possible machine output, grinding of the feedstock is to occur at the maximum possible temperature. If in so doing the material-dependent temperature limit is exceeded, the feedstock softens and begins to melt with the result that individual particles agglomerate and thereby the particle size and particle distribution of the comminuted material are no longer within the desired range. Or a thermal decomposition of the feedstock occurs, whereby the particles heated above the temperature limit bake onto the machine parts and particularly onto the comminuting tools, so that both the machine output and the quality of the end product suffer as a result.
Reducing the output of comminuting devices is a known approach to prevent thermal damage to the feedstock during the comminution thereof. In this way, less comminuting work is done per unit time, thus producing less excess heat. It must be accepted in this regard, however, that the comminuting device is not operated at full capacity, which is contrary to the fundamental requirement of an economic operation of such devices.
This problem is compounded in fine grinding, because it was found that the finer the end product is to be, the more comminuting work has to be done and the greater the heat input into the feedstock.
Increasing the cooling effect by increasing the amount of the self-generated air of a conventional comminuting device, in order to be able to remove the additional heat, is possible only within narrow limits, because the amount of the self-generated air determines the flow velocity and thereby the residence time of the grinding stock in the grinding zone and thereby also the fineness of the end product.
During the fine grinding of certain materials such as, for example, rubber, caoutchouc, and elastomers, there is, moreover, the problem that grinding of the feedstock is not readily possible because of its elastic properties. For this reason, in these cases the transition has already been made to cool the feedstock to temperatures far below the freezing point by adding a coolant. The associated embrittlement of the feedstock then allows comminution of the embrittled particles by means of breaking (cryogenic comminution). Because the cooling of the feedstock to the necessary low temperatures requires considerable amounts of coolant, this type of comminution is very cost-intensive. Moreover, cryogenic comminution produces an end product whose particles are characterized by a cubic shape with a relatively smooth surface. These material properties are disadvantageous, however, for some uses of the end product, particularly if forming a connection with other materials as intimate as possible is a required ability.
DE 197 15 772 C1 discloses a method and a device for preparing plastic waste products. The waste products are first subjected to a precomminution in a shredder and the raw grinding stock obtained thereby is stored temporarily in a storage tank. The raw grinding stock is then fed into an impact mill for further comminution. The substances present in the grinding stock are separated at subsequent stations and supplied as a secondary raw material for recycling. It has also been recognized here that frictional heat is formed due to the grinding process in the impact mill; such heat is in fact desirable for drying the grinding stock but nevertheless should not be too hot. In order to keep the temperature of the grinding air in a range between 70° C. and 92° C., it is proposed to supply water to the grinding air in the feed to the impact mill.
Further, DE 37 08 914 A1 discloses an impact mill for comminuting grinding stock with a rotor, equipped with impact tools and disposed within a housing. The grinding stock is supplied to the rotor via a supply opening and after it is comminuted, it is fed into a separating device. The metered addition of grinding stock to the impact mill occurs depending on the power consumption of the drive for the rotor by scaling back the feed when the drive's current consumption is too high.